Considering a pick-and-mix model of dark matter

What if there are more than WIMPs on the Universe's dark side?

Dark matter is the physical mystery of our time. We know from many different results that a fair percentage of the Universe consists of matter that doesn't seem to interact in any way other than gravity. But with every statement like that, there is a limit: our measurements are only so sensitive, which leaves space for dark matter to interact. If it does, the interactions have to be weak. Hence, most physicists think that dark matter consists of weakly interacting massive particles (WIMPs).

Not all candidate dark matter particles fit with experimental data, so they can be discarded, right? Maybe not, according to physicists at Harvard. "Wait," I hear you say, "data is king, so they can't be right." But they actually can, because when data gets turned into models, assumptions often get involved. The underlying reason for kicking dark matter proposals out is the assumption that the entirety of dark matter consists of a single type of particle (or, more accurately, many different types of particles with very similar properties). But who is to say that dark matter doesn't consist of a mix of different particles with vastly different properties?

Setting off for the shores of a universe that has a complex distribution of dark matter particles begins with a journey of small steps. In this case, the researchers considered a population that has a mixture of standard WIMPs and a population of dark matter that doesn't interact with ordinary matter, but does interact with itself. Think of this second population as a mirror universe of protons and neutrons that can clump together through electromagnetic-like forces but are almost untouchable by ordinary matter. A universe composed of these two populations of dark matter will look very different from one composed of only WIMPs.

There is some room to produce a more familiar universe—as long as there isn't too much of this new dark matter, the universe won't mind—so the researchers calculated just how much dark matter could be strongly interacting. They found that this comes out to about five percent, which is roughly the amount of ordinary matter that is in the Universe. An interesting coincidence, if nothing else.

Interacting dark matter has observational consequences, though, which is what makes it interesting. In a spiral galaxy like the Milky Way, it will accumulate in a disk. That means that our dark matter detection experiments, like Fermi and PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics), should see more dark matter than expected. And although the results are uncertain, both Fermi and PAMELA have reported possible dark matter signals that are stronger than expected.

But the dark matter disk doesn't have to be oriented in the same plane as ordinary matter. So the expected excess experienced by an observer in a galactic disk can vary from zero upward to some maximum amount, depending on the relative orientation of the ordinary matter and dark matter disks, adding yet another free parameter into the mix. This, I think, is a bad thing.

This richer model of dark matter may really come into its own, though, in explaining some odd results. For instance, the bullet cluster observations can be explained by either standard WIMP dark matter or by a mixture of different dark matter components. But the Abel 520 cluster observations are not so easily explained by WIMPs alone, though a more complex version of dark matter might make sense of it.

Unfortunately, we don't yet know. This paper was more about the bounding possibilities: how much freedom is there in the mixture? We would still need a more developed model before dark matter distributions can be compared to real astronomical observations. But this model is timely, because the Planck and Gaia missions will shine an awful lot of light on dark matter.

So where does that leave us? Well, we still don't know what dark matter is, but this paper shows that the possibilities are much wider than the ones we've generally considered. In some ways, that's a lot of fun, because it opens data to new interpretation and should result in a more consistent model. On the downside, our limited observational power may mean that we have to wait a long time to arrive at a unique description of dark matter.

the researchers calculated just how much dark matter could be strongly interacting. They found that this comes out to about five percent which is roughly the amount of ordinary matter that is in the Universe

Ordinary matter comprises 5% of what? Did you mean that ordinary matter is 5% of ordinary + dark matter?

So here's an interesting thing to think about... What if this 5% dark matter interacts with itself in all the same ways our "regular" matter interacts with itself. Will there then be beings made up of that 5% stuff wondering about the 5% stuff we inhabit?

EDIT: moving away from sci-if philosophizing, is it possible there is basically "dark symmetry"? Dark versions of all the particles in our standard model. 16 dark versions of the 16 standard model particles. Within each set are the same interactions as the standard model. But interactions between the two sets only occur as mediated by the Higgs, which has the same interaction with particles from both sets.

the researchers calculated just how much dark matter could be strongly interacting. They found that this comes out to about five percent which is roughly the amount of ordinary matter that is in the Universe

Ordinary matter comprises 5% of what? Did you mean that ordinary matter is 5% of ordinary + dark matter?

It comes out to pretty much the same thing. The amount of ordinary matter is a small percentage of the total, so adding it to the total doesn't change the total much. Either way the percentage would only be different by about 0.5% or so.

Need to open that picture up in Photoshop to check out if there is some sort of "hidden matter" in that too suspiciously dark frame. Maybe some hidden messages to the NSA or maybe messages from subspace aliens to Chris Lee or maybe....

At least this time the whoosh sound wasn't quite as loud as the last one.

"But the dark matter disk doesn't have to be oriented in the same plane as ordinary matter."

It seems to me that reasoning from first principles the disks would be more likely to be aligned than not. The disk's orientation in effect records the net local angular momentum in that area of space. The net local angular momentum only varies because the uneven distribution of mass energy after the big bang caused initial gravitational imbalances that resulted in gravitational collapses that concentrated the local angular momentum variations in structures like galaxies.

This process is pretty much entirely driven by gravity, so it ought to affect dark matter and ordinary matter identically, and ought to result in aligned disks as long as the initial post-big-bang fluctuations in the distribution of dark matter initially followed the same pattern as that of ordinary matter, since the angular-momentum-concentrating process works the same for both ordinary and dark matter, and should start (I would think) from the same initial pattern of uneven distribution.

"Dark matter" is the black box that "scientists" throw into that equation where it says "a miracle occurred".

That's my opinion as well.

Is this pick-and-mix scenario the beginning of ptolemaic epicycles?

But you're missing the point. Epicycles were the wrong solution, but there was a problem they were addressing that, at the time, no one had a better solution for. The whole point of the term "dark matter" is that we know there's a problem and aren't sure of the exact solution. Right now the point that it's just something scientists threw in is true, but I fail to see why that should result in a critical tone.

A circle with epicycles is analogous to the series expansion of an ellipse. You can describe an ellipse directly, or as a circle with infinite epicycles to get everything just right. Every so often, data would improve to the point that effects of the next term in the expansion were relevant, and so more epicycles were added. Kepler's great genius is recognizing "Ah, these epicycles are converging on ellipses!" not "Ah, epicycles are total bullshit!" My point is, epicycles are empirically motivated and correct. Astronomers in the 1500 and 1600s knew far more about planetary orbits than I (and, if you're comparing epicycles to dark matter, you) do. Get off their case.

moving away from sci-if philosophizing, is it possible there is basically "dark symmetry"? Dark versions of all the particles in our standard model. 16 dark versions of the 16 standard model particles. Within each set are the same interactions as the standard model. But interactions between the two sets only occur as mediated by the Higgs, which has the same interaction with particles from both sets.

There is a part of me which really likes the idea of dark symmetry. Then there would be a 'light' side and a... you get the picture

Quote:

...the Planck and Gaia missions will shine an awful lot of light on dark matter.

Kepler's great genius is recognizing "Ah, these epicycles are converging on ellipses!" not "Ah, epicycles are total bullshit!"

Thanks for this. I'd like to add that the reason then that ellipses are preferred even though they're mathematically idential is because, like Occam's Razor says, epicycles are more complicated and provide no additional explanatory power. People often forget that Occam's Razor (which is often introduced in school at the same time as Kepler, or at least was for me) doesn't say that the universe is probably simple. It says don't make it more complicated than it has to be if there's no benefit. If two hypothesis make the same predictions, you pick the simpler one*.

Dark Matter makes very different predictions than theories without it. And the proposed variants covered in this article makes different predictions than pure-WIMP DM models. Which means that with sufficient data we could distinguish and determine which does a better job of explaining the universe. Until you have that data, though, you have to keep all the possibilities in mind.

This is nothing like epicycles.

* Though even if they're mathematically the same, it can still be important to keep them both in mind, because they may provide different ways of thinking about the same problem and one might prove more useful in crafting the next hypothesis. In the case of planetary motion this turned out to also be ellipses, since it was once he realized the motion was elliptical that he was able to deduce additional laws about planetary motion, and the question of why it should be ellipses helped Newton develop his law of gravity.

A circle with epicycles is analogous to the series expansion of an ellipse. You can describe an ellipse directly, or as a circle with infinite epicycles to get everything just right. Every so often, data would improve to the point that effects of the next term in the expansion were relevant, and so more epicycles were added. Kepler's great genius is recognizing "Ah, these epicycles are converging on ellipses!"

Interesting - I did not know that epicycles survived into heliocentric models. There are some animated models here that illustrate Copernicus' use of epicycles.

A better comparison to dark matter would be phlogiston. Given observational evidence, a theory was worked out which would explain the observed behavior. But as scientists began trying to detect it directly, they discovered that it would have to possess contradictory properties, so it came to be discredited.

We're in the "trying to detect" stage with dark matter now. Whether we'll eventually detect it, or whether we're looking for something that doesn't exist, remains to be seen.

Personally I think the best comparison for Dark Matter is to two separate but related mysteries of the past: The odd orbits of Uranus and Mercury.

Here were two cases where the observed dynamics could not be explained by our best theory of gravity of the time and the known masses. And in both cases there were two obvious paths to a solution: First, we could assume our theory of gravity, which had been very successful so far, was correct which would imply that there was an as yet known mass disturbing the orbits. Second, it could be that our theory of gravity is not correct and a better theory would explain the orbits without any new masses required.

In the case of Uranus, it was the first solution that was correct, and we discovered the planet Neptune exactly where it was predicted to be. In the case of Mercury, there was no unknown mass, and it was only once a new theory of gravity came along that we were able to understand it.

Right now we're in the stage before figuring out if General Relativity is correct and the implied mass exists, or if we need a new theory of gravity. Though it's a very tall order to figure out how you could just modify gravity and explain some of the observations that have been made.

Note that the Dark Matter hypothesis, which is essentially just the "Neptune solution" regardless of what form that mass takes, is different from the WIMP hypothesis, which is one of the possibilities for Dark Matter that we are trying to detect. We could fail to find WIMPs yet still have the available evidence showing Dark Matter as a more likely option than modified gravity.

Personally I think the best comparison for Dark Matter is to two separate but related mysteries of the past: The odd orbits of Uranus and Mercury.

Here were two cases where the observed dynamics could not be explained by our best theory of gravity of the time and the known masses. And in both cases there were two obvious paths to a solution: First, we could assume our theory of gravity, which had been very successful so far, was correct which would imply that there was an as yet known mass disturbing the orbits. Second, it could be that our theory of gravity is not correct and a better theory would explain the orbits without any new masses required.

In the case of Uranus, it was the first solution that was correct, and we discovered the planet Neptune exactly where it was predicted to be. In the case of Mercury, there was no unknown mass, and it was only once a new theory of gravity came along that we were able to understand it.

Right now we're in the stage before figuring out if General Relativity is correct and the implied mass exists, or if we need a new theory of gravity. Though it's a very tall order to figure out how you could just modify gravity and explain some of the observations that have been made.

Note that the Dark Matter hypothesis, which is essentially just the "Neptune solution" regardless of what form that mass takes, is different from the WIMP hypothesis, which is one of the possibilities for Dark Matter that we are trying to detect. We could fail to find WIMPs yet still have the available evidence showing Dark Matter as a more likely option than modified gravity.

A circle with epicycles is analogous to the series expansion of an ellipse. You can describe an ellipse directly, or as a circle with infinite epicycles to get everything just right. Every so often, data would improve to the point that effects of the next term in the expansion were relevant, and so more epicycles were added. Kepler's great genius is recognizing "Ah, these epicycles are converging on ellipses!"

Interesting - I did not know that epicycles survived into heliocentric models. There are some animated models here that illustrate Copernicus' use of epicycles.

In fact astronomers objected to Copernicus' model partly because it required more epicycles than a geocentric one! It was because the elliptical nature is more manifest in such a model.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.